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The two 27-inch iMacs use quad core Intel Core i5 processors out of the box. The cheaper one has a 3.2GHz chip, while the more expensive model runs at 3.4GHz. These are up from 2.9GHz and 3.2GHz on last year's 27-inch iMacs, also quad core Intel Core i5s.

At times of high processing needs, power can be redistributed using the chips' Turbo Boost feature, temporarily boosting the clock speeds to 3.6GHz and 3.8GHz respectively.

If you choose the high-end 27-inch model, you can upgrade the processor to a 3.5GHz Core i7 for £190 (about US$300, AU$320). The key advantage of this processor is it offers Hyper Threading, whereby each of the processors' four cores can run two threads for eight virtual cores, a feature the quad core Core i5 CPUs lack.

For graphics, the new large-screen all-in-ones have Nvidia GeForce GT 755M with 1GB video memory and Nvidia GeForce GTX 775M with 2GB respectively, up from GeForce GTX 660M with 512MB and GeForce GTX 675MX with 1GB.

Hard drives remain the same; 1TB out of the box, using the faster 7,200rpm drives instead of the 5,400rpm models seen in the 21.5-inch iMacs. If you order from Apple's online store, you can upgrade to a 3TB hard drive, a 1TB or 3TB Fusion Drive, which combines a hard drive with a solid state section for regularly-accessed data, or a 256GB, 512GB or 1TB solid state drive.

Memory remains the same, at 8GB, but again you can configure your iMac for 16GB or 32GB if you order online. Unlike the 21.5-inch models, the memory stick slots in a 27-inch iMac are user-accessible, so it's possible to upgrade it yourself after purchase.

Don't expect to upgrade too much else, though. The iMac was never designed for user-upgradeability, and the slimline form factor introduced in 2012 is especially problematic as the screen is glued to the bezel instead of being held in place by magnets, making it difficult for even specialists to get at its components.

For wireless connectivity, the new 27-inch iMacs offer Bluetooth 4.0, and also WiFi ac, which is up to three times as fast as the previous generation's WiFi n. You need to be connected to a Wireless ac-compatible router, though.

The 2013 large-screen iMacs cost £1,599 (about US$2,60, AU$2,670) and £1,749 (about US$2,840, AU$2,900) respectively, which is £100 (about US$160k, AU$170) more for the entry-level model and £50 (US$80, AU$80) extra for the high-end release. The 21.5-inch models are also up by £50 each.